A conventional module for housing an air cushion restraint system in a vehicle instrument panel has a cover or deployment door which attaches to a housing or canister in which an air bag and inflator are housed. Most commonly, a plurality of threaded studs extend from the canister about its mouth and the cover has corresponding complementary holes which mate with the studs. A reinforcement plate is then mated with studs and threadingly retained by nuts to trap the cover between the plate and canister. By trapping the cover to the canister along at least a pair of primary opposed edges, the cover is securely retained to the canister even during deployment of an air bag through rip seams formed in the cover. Preferably, a rip seam pattern defines a pair of opposed deployment doors in the cover with edge hinges for integrally retaining each door in hinged relation with the canister during air bag deployment. However, such systems require a large number of separate pieces when assembling the cover to the canister which increases assembly time and component expense. Use of a plurality of separate fasteners and support bracketry also complicates the assembly and repair. Furthermore, retention of a cover to a canister with threaded studs often requires checking for inadvertent cross-threading of the studs during assembly in order to verify proper assembly. Furthermore, the presentation of an air bag can be modified when the cover separates from the canister during deployment which varies the restraint performance from a designed and expected performance.
An alternative method for retaining a cover to a canister includes mounting the canister to a vehicle instrument panel support structure and separately fastening the cover to adjacent surrounding instrument panel structure such that the cover and canister are engaged in abutment but are not actually physically retained together.
In another version, one edge of an air bag module cover is retained to a canister by trapping it against the canister with threaded fasteners and a reinforcement plate. The opposite edge is then snap-fit engaged with the corresponding opposite edge of the canister in order to facilitate assembly and to reduce the number of parts in the system. However, such a system can not be totally snap-assembled to a canister in a quick and efficient manner so that separate fasteners and plate reinforcement assemblies are not needed when retaining the cover to the canister.
An even further version utilizes a cover which snap-fit engages to a housing with tabs to eliminate these fasteners. However, the entire face of the cover separates from the tabs along one entire edge, and a hinge is formed adjacent the tabs on the opposite edge such that the entire cover face forms a pivoting door for deployment of an air bag. Such a design does not fully circumferentially retain the cover to the canister during air bag deployment.
Further methods are available for retaining a cover to a canister when constructing a passenger air bag module, for example, a plurality of rivets can be used to trap a cover to a canister about its mouth. When rivets are used, there is often concern over the quality of the assembled rivets, as well as the integrity of the riveted assembly, namely its reliability and strength when subjected to forces and loads generated during an air bag's deployment through the cover.
Additional complications exist when utilizing fasteners and reinforcing plates to secure an air bag cover to a canister. For example, threaded fasteners are susceptible of loosening from vibration as a vehicle travels over a bumpy road and therefore require use of a thread binding agent or a lock washer to prevent loosening. This further increases the number of parts, complicates the assembly, and adds to cost and time required for assembly. As a result, such systems for attaching a cover to a canister tend to be less feasible when constructing air bag modules for use in high volume and low cost applications.